Power control is important for capacity and efficiency in mobile WCDMA systems. For example, if a mobile transmitter/receiver unit, such as a mobile radio terminal, is located close to a base station transmitter/receiver, the power level of signals, which are wireless transmitted over an air interface to the mobile unit from the base station, in absence of adjustment, would be comparatively high. This could interfere with transmissions to other mobile units located farther away from the base station. Conversely, the signal power of transmissions from a mobile unit, which was located far from the base station, could be comparatively weak. Accordingly, it has become common practice to provide a transmission power control in such wireless communication systems.
Currently, power control is accomplished by estimating the signal to interference ratio (SIR) of received signals. If the SIR of a signal received at a mobile unit is lower than a threshold value, a command or adjustment signal is sent to the transmitting base station to increase transmission power. The command is sent on the reverse link of the communication system, which could either be the uplink or the downlink, depending on which link is controlled. If the estimated SIR is higher than the threshold value, a command to decrease transmission power is sent, or vice versa.
More specifically, as is well known to those skilled in the art, the signal to interference ratio is a crucial quantity in the inner loop power control. In a typical power control system, see FIG. 1B, the outer loop sets the reference target SIR and the inner loop adjusts the transmitted power such that the estimated SIR agrees with the reference or target SIR. The inner loop controls the communication between a base station and a mobile user equipment and vice versa. This is achieved by giving power up or power down commands, that is, TPC (Transmit Power Control, as mentioned above) equals one or zero respectively. To each transport block there is a cyclic redundancy check (CRC). If the decoded CRC is determined to be correct, the CRC error flag (CRCef) is set to zero, otherwise to one. Filtering or averaging the CRCef in another appropriate way obtains an estimate of the block error rate (BLER). Usually the outer loop consists of an appropriate regulator that adjusts the reference SIR value depending on the discrepancy between the reference BLER and the estimated BLER. The purpose of the power control is to keep the estimated BLER as close as possible to the reference BLER.
The inner loop runs usually at such a frequency that a new SIR estimate is produced every slot. In the 3GPP specification it is suggested that the pilot symbols be used for the SIR estimation. The user equipment (UE), such as a mobile radio terminal, is required to produce a TPC command within 512 chips beginning from the first arrived propagation channel path, see FIG. 2. The relative power difference between the data symbols and the pilot symbols is known. The transmitted power change is always done at the beginning of the pilot field. However, using only the pilots in estimating the SIR produces very noisy estimates, which in turn produces a large variance on the transmitted powers.
It is not desired to have such a variance of transmitted powers as both the UE and the base-station benefit from a low variance in the transmitted power. With a low variance of transmitted powers, e.g. a lower average transmitted power can be achieved and thus additional capacity in the base-station and less power consumption at the UE is achieved.
U.S. Pat. No. 6,070,086 discloses the use of the data bits as well as the pilots for estimating the SIR. The data bits are added coherently, whereby the sent bits have to be known. In U.S. Pat. No. 6,070,086 hard decisions are made on the data bits. This implies that few mistakes in decoding the sent data bits are made, if the channel quality is good. Thus they can consequently be useful in estimating a more precise SIR. However, the contrary is true if the channel is of poor quality due to e.g. noise or other disturbances.
Furthermore, in the above mentioned state-of-the-art method the power estimates of the pilot and data symbols are given equal weights and the sent TPC command is always assumed to be received correctly. In practice this is not always the case, thus resulting in inaccurate SIR estimation. It is believed that power is changed in one direction, i.e. up or down depending on the TPC data, but in reality it is changed in the other direction, because the received TPC data has been modified by transmission errors. This results in an inaccurate SIR estimation and causes also high variance of the transmitted power, as mentioned above.
Therefore, a need exists for a robust and precise SIR estimate under conditions with noise disturbed transmission channels and unknown contents of the transmitted data.